Such a metering device has become known from EP-A-2-231,513. This prior-art anesthetic metering device consists of a reservoir, from which liquid anesthetic is pressed into a heated intermediate container by a delivery pressure. The heating temperature and the internal pressure of the intermediate container are adjusted to one another so that the liquid anesthetic evaporates immediately after entry into the intermediate container and occurs in this only as a gaseous anesthetic. Via a cyclically operating metering valve in the outlet line, the pressurized anesthetic vapor is fed into a carrier gas line, through which, e.g., an oxygen-laughing gas mixture is sent to an anesthetic apparatus. Corresponding to the vapor pressure in the intermediate container, the degree of opening of the metering valve and the volume flow of the carrier gas per unit time, a corresponding anesthetic concentration value becomes established in the carrier gas stream.
One disadvantage of the prior-art anesthetic metering device is the fact that in the case of insufficient heating of the intermediate container or malfunction of the heating device, liquid anesthetic may enter the carrier gas stream and lead to a considerable, undesired increase in concentration there. Liquid anesthetic may enter the carrier gas stream especially if the metering valve is jammed in the open state due to a malfunction. Another disadvantage is the fact that complicated means are required for a simple control of the amount of liquid anesthetic that is delivered into the intermediate container from the reservoir and evaporates in it the intermediate container. Even a slight change in the amount of anesthetic delivered from the reservoir into the intermediate container leads to a drastic change in the vapor pressure at a given temperature there.
In the prior-art metering device, the pressure of the evaporated anesthetic in the intermediate container is exactly equal to the delivery pressure in the reservoir. Therefore, this pressure must be exactly preset in a complicated manner. In the case of low-boiling anesthetics, which boil even at room temperature, this pressure control in the reservoir is extremely difficult, because the vapor pressure that develops may be on the same order of magnitude as the desired delivery pressure.